Several lines of evidence indicate that myocardial ischemia, due to alterations in blood flow, may play a central role in the national history of hypertrophic cardiomyopathy. Myocardial imaging with the isotope Thallium-201 offers a noninvasive means of asessing blood flow. We undertook the present study to determine the relative prevalence of perfusion abnormalities across a wide spectrum of patients with hypertrophic cardiomyopathy. Accordingly, 72 patients ranging in age from 12 to 69 years underwent Thallium-201 emission computed tomography (ECT) in conjunction with treadmill exercise. Fifteen of the patients had resting depression of left ventricular function as manifested by a reduced ejection fraction. Fourteen of these 15 patients demonstrated fixed or only partially reversible perfusion abnormalities consistent with underlying areas of myocardial fibrosis and/or severe ischemia. Of the remaining 57 patients with normal or hyperdynamic left ventricular function, 48% demonstrated perfusion abnormalities predominantly of the reversible type. These latter defects are consistent with dynamic, stress-induced ischemia. These results extend and confirm previous observations concerning Thallium perfusion defects in patients with hypertrophic cardiomyopathy. The fixed or only partially versible defects seen in patients with resting left ventricular dysfunction most likely do represent areas of underlying scar and hence provide an explanation for the associated impairment in contractile function. The reversible defects observed in the other patient subgroup reflect a more dynamic process, which, if allowed to continue, may eventuate in either the progression of symptoms, the development of an arrhythmic complication, or the gradual replacement of myocardium by a process of necrosis and infarction leading eventually to left ventricular dysfunction and congestive heart failure.